1. Field of the Invention
This invention relates to a signal checking system for accurately and reliably detecting appearance of an erroneous output or multiple outputs due to erroneous selection or multiple selection by a converter which is adapted for converting a coded input signal consisting of a plurality of bits into one of a plurality of individual single output signals.
2. Description of the Prior Art
In a hardware system including a converter adapted for converting a coded input signal consisting of a plurality of bits into one of a plurality of individual single output signals, a plurality of terminal equipments (controlled systems) are frequently directly controlled by the individual single output signals appearing from the converter as a result of the conversion. Therefore, when the element or elements of the converter fail to properly operate, an output signal different from a specific output signal will appear from the converter due to erroneous selection or a plurality of output signals will appear from the converter due to multiple selection in response to the application of an input signal instructing one of the terminal equipments to be selected. Execution of control while ignoring this unusual selection will result in erroneous control of the specific terminal equipment, and undesirable system-down will be given rise to.
A so-called loop checking method, for example, has been generally employed to detect appearance of an undesirable output or outputs due to erroneous selection or multiple selection by a converter which is adapted for converting a coded input signal consisting of a plurality of bits into one of a plurality of individual single output signals. In such a prior art method, a signal sent back from a controlled station is compared in a control station with a signal sent out already and stored in a memory in the control station which has the function of detecting the presence of any error between these signals. Thus, this prior art method has been defective in that an unusual situation occurring in a converter in the control station cannot be quickly dealt with and remedied. Further, the entire loop checking system is included in the trouble detection range of the control station according to the prior art method. Thus, when, for example, the controlled station is quite remote from the control station or a switching system is present therebetween, all these circuits or the signal transmission paths and signal transmitting and receiving circuits on the receiving side are included in the trouble detection range of the control station. Therefore, the prior art loop checking method adapted for error detection on the basis of the loop checking in the control station has also been defective in that the trouble detection range is very wide resulting in difficulty of quick detection of a faulty element or part.